Ultrasonic surgical instruments are known which include ultrasonic surgical blades. A handpiece of a known ultrasonic surgical instrument includes an ultrasonic transducer which is powered by an ultrasonic generator through a cable. An ultrasonic transmission rod of the instrument has a first end and a second end. The first end of the ultrasonic transmission rod is operatively connected to the ultrasonic transducer. The second end of the ultrasonic transmission rod is connected to, or is an integral proximal extension of, an ultrasonic surgical blade. The ultrasonic surgical blade is activated by the ultrasonic transmission rod providing longitudinal ultrasonic vibration to the blade. Known blade shapes include straight blades and curved blades and include blades that are symmetric and blades that are asymmetric about a longitudinal axis or about a curved centerline of the blade. Known blades are designed to provide substantially purely longitudinal ultrasonic vibration along the blade. Exemplary devices are described in U.S. Pat. Nos. 5,322,055 and 6,325,811, the contents of which are incorporated herein by reference.
A known ultrasonic surgical blade is a cylindrical blade which has a distal tip, a most-distal vibration node (a vibration node being a point of substantially zero displacement), and a second most-distal vibration antinode (a vibration antinode being a point of maximum displacement relative to all other points in a half wave), wherein the most-distal vibration antinode is the distal tip. Longitudinal ultrasonic vibration of the blade generates motion and heat in the contacted tissue, wherein the heat primarily provides the means for the blade to cut and/or coagulate patient tissue.
The amplitude of the longitudinal ultrasonic vibration of a conventional ultrasonic surgical blade is zero at the most-distal vibration node and rises along a cosine curve to a maximum at the distal tip of the blade. Typically, an amplitude of at least 50% of the maximum amplitude is required for effective cutting, and the distance from the distal tip to such 50% point on the blade can be considered to be the effective cutting length of the blade. The blade is not considered useful beyond its effective cutting length. The effective cutting length is about 13 mm for a straight cylindrical titanium rod at a resonant frequency of about 55.5 kHz and an amplitude of at least 55 microns.
Still, scientists and engineers continue to seek improved ultrasonic surgical blades.